1. Field of the Invention
The present invention relates to a hydrodynamic bearing structure having hydrodynamic (dynamic-pressure) grooves and to a rotating apparatus having the hydrodynamic bearing structure, a deflection scanning apparatus, a laser beam printer, an image forming apparatus, and a rotating apparatus of hard disk. More particularly, the invention concerns a hydrodynamic bearing apparatus for rotationally supporting a rotary polygon mirror or the like for high-speed scanning with a light beam in laser beam printers, bar code readers, etc., a method of producing the hydrodynamic bearing apparatus, and a deflection scanning apparatus using the hydrodynamic bearing apparatus.
2. Related Background Art
The deflection scanning apparatus used in the laser beam printers, bar code readers, etc. is constructed to implement deflection scanning with a light beam such as a laser beam or the like by use of a rotary polygon mirror rotating at a high speed. In the image forming apparatus such as the laser beam printers or the like, the scanning light yielded by the rotary polygon mirror is focused on a photosensitive body on a rotary drum to form an electrostatic latent image thereon, the electrostatic latent image on the photosensitive body is developed into a toner image by a developing device, the toner image is transferred onto a recording medium of a recording sheet, and the recording medium with the toner image is transferred to a fixing device to heat the toner on the recording medium to fix it, thereby performing printing.
The deflection scanning apparatus of this structure has been increasing its operation speed and accuracy more and more in recent years and, in response thereto, the hydrodynamic bearing apparatus of a non-contact type to permit low-noise and high-accuracy rotation has been and is employed in the bearing part of the rotary polygon mirror.
FIG. 1 is a schematic, cross-sectional view showing a hydrodynamic bearing unit according to a conventional example. The bearing unit has a shaft 102, which is arranged to rotate integrally with a rotary polygon mirror 101 having a plurality of reflecting facets 101a, and a sleeve 103, in which the shaft 102 is fitted so as to be rotatable. The sleeve 103 is integral with a bearing housing 104. Fixed at the lower end of the sleeve 103 is a thrust pad 106 provided with a spherical portion 106a for supporting the lower end of the shaft 102 in the thrust direction. A flange 107 is fixed to the shaft 102 at the upper part thereof. The rotary polygon mirror 101 is pressed against the upper surface of the flange 107 by an elastic press mechanism 108 including a presser spring, etc. to form an integral structure therewith, so as to rotate together with the shaft 102.
A yoke 109a holding rotor magnets 109 is fixed to the peripheral part of the flange 107 and the rotor magnets 109 are opposed to al stator coil 110 on a base plate 105 fixed to the bearing housing 104. When the stator coil 110 is energized by a driving current supplied from a driving circuit (not shown), the rotor magnets 109 rotate at about 10,000 or 20,000 rpm together with the shaft 102 and rotary polygon mirror 101.
A fluid membrane is created between the sleeve 103 and the shaft 102 with rotation thereof, thereby constituting a hydrodynamic bearing which rotationally supports the shaft 102 in a non-contact state by the dynamic pressure of the fluid membrane. First dynamic-pressure generating grooves 102a and second dynamic-pressure generating grooves 102b are cut with some spacing in between and in the stated order in the upward direction from the lower end of the shaft 102 in the peripheral surface of the shaft 102. Shallow grooves (not shown) forming a hydrodynamic thrust bearing are also provided at the position facing the lower end of the shaft 102, in the upper surface of the thrust pad 106.
With rotation of the shaft 102, a fluid 111 such as oil or the like present in a bearing clearance between the shaft 102 and the sleeve 103 is pulled into the central part of each dynamic-pressure generating groove 102a, 102b to generate a high-pressure region there. Such high-pressure regions work to support the shaft 102 while maintaining the non-contact state in the radial direction between the shaft 102 and the sleeve 103. Because of this non-contact rotation, the above bearing structure has advantages of capability of yielding properties such as lower noise, higher rotation accuracy, etc. than sliding bearings accompanied by metal contact and capability of reducing the size and cost in terms of the number of assembled parts as compared with, for example, rolling bearings, etc.
Used heretofore as a base oil of a lubricant for such hydrodynamic bearing apparatus were oils having a molecular weight distribution, such as mineral oils, synthetic hydrocarbon oils, silicone oils, perfluoro oils, etc., single-component oils such as diester oils, polyol ester oils, and so on, and mixed oils thereof.
For rotating the hydrodynamic bearing apparatus, there were desires for decrease in viscosity of the mixed oils, and the components of the mixed oils and percentages of the components were determined for the purpose of adjusting the viscosity. Describing in more detail, because the load carrying capacity and bearing rigidity of the hydrodynamic bearing apparatus are proportional to the viscosity of the lubricant used, it is necessary to select the lubricant of the viscosity matching with the required characteristics of an equipment provided with the hydrodynamic bearing apparatus. In addition, it is necessary that the change in the viscosity is small for the selected lubricant during long-term use.
It is relatively easy to adjust the viscosity to an arbitrary value for the oils such as the mineral oils, synthetic hydrocarbon oils, silicone oils, perfluoro oils, etc. used heretofore as base oils of lubricants. Since these base oils themselves have a broad molecular weight distribution, low-molecular-weight components thereof will volatilize during use over an extended period of time as a lubricant of the hydrodynamic bearing apparatus, so as to vary the viscosity gradually. More specifically, the percentages of the low-molecular-weight components in the lubricant will lower under the long-term use of the hydrodynamic bearing apparatus, so that the viscosity of the lubricant will become higher. Because of such a change in viscosity with the elapse of time, it is difficult to keep the load carrying capacity and bearing rigidity constant throughout the long-term use.
In cases where the so-called single-component oil such as diester oils, polyol ester oils, purified and separated synthetic hydrocarbon oils, for example, PAO (poly-xcex1-olefins), etc. is used, without carrying out the step of adjusting the viscosity by addition of another oil, because the viscosity of the base oil is perfectly dependent upon the molecular structure of the single-component oil itself, molecular design is required for adjusting the base oil to an arbitrary viscosity matching with the required characteristics of the equipment provided with the bearing apparatus and it is thus hard to obtain a desired single-component oil.
As to the conventional mixed oils, it is relatively easy to adjust the viscosity thereof to an arbitrary value by combining a low-viscosity-component oil with a high-viscosity-component oil, as compared with the single-component oils. Since the conventional mixed oils were prepared while focusing on the adjustment of viscosity, they demonstrated variation in the composition of the base oil because of volatilization of highly volatile components or low-molecular-weight oils during long-term use as a lubricant of the hydrodynamic bearing apparatus, as described above. As a consequence, a change in viscosity occurred with the elapse of time.
The present invention has been accomplished in view of the unsolved problems of the prior art and an object of the present invention is to provide a hydrodynamic bearing apparatus capable of making great contribution to stabilization of bearing performance and to improvement in rotation accuracy by use of the lubricant having an arbitrary viscosity matching with the required characteristics of the equipment provided with the bearing apparatus (hereinafter referred to as xe2x80x9cbearing characteristicsxe2x80x9d) and demonstrating little change in viscosity even under long-term use, and a deflection scanning apparatus using it.
In order to accomplish the above object, a hydrodynamic bearing apparatus of the present invention comprises a shaft member and a sleeve member fitted so as to be rotatable relative to each other, a lubricant filling a bearing clearance between the shaft member and the sleeve member, and a rotary member integrally coupled to the shaft member or the sleeve member, wherein the lubricant comprises a base oil comprising a mixed oil consisting of a plurality of components the vapor pressures of which are equal or close to each other.
The difference in vapor pressure of the plural components is preferably not more than 5xc3x9710xe2x88x924 Torr and more preferably not more than 5xc3x9710xe2x88x925 Torr in the temperature range of not less than 0xc2x0 C. and not more than 60xc2x0 C.
In the present invention, the base oil of the lubricant is a mixed oil prepared in such a viscosity as to match with the bearing characteristics by combining a plurality of components of different viscosities. The viscosity change of the lubricant due to volatilization of a specific component is prevented by selecting the combination of the base-oil components such that the vapor pressures of the plural components are equal to each other or such that the difference in vapor pressure thereof is not more than 5xc3x9710xe2x88x924 Torr and, describing in further detail, not more than 5xc3x9710xe2x88x925 Torr.
As a consequence, the hydrodynamic bearing apparatus can be realized without a change in viscosity of the lubricant even in case of long-term use, while maintaining high rotation accuracy and stable bearing performance.
When the hydrodynamic bearing apparatus of this structure is applied to the bearing part of the rotary polygon mirror, contribution can be made to attainment of a longer life and higher performance of the deflection scanning apparatus.